The present invention relates to a high power semiconductor device, and particularly to a static induction type semiconductor device used as a power transistor for producing a high current density.
A static induction type semiconductor device has been used as one of the semiconductor devices for controlling a large current. The static induction type semiconductor is disclosed in a U.S. patent application Ser. No. 181102 filed on Apr. 13, 1988 and, in pp. 40-44, Nikkei Electronics, 1976.9.20. In the high current density region of such semiconductor apparatuses for controlling a large current, the ratio of main current to be controlled, to control current (called current amplification factor), should be large.
An example of the conventional static induction type semiconductor device (which is called SIT) is described.
Examples of conventional static induction transistors of the surface gate type (called surface-gate-type SIT hereafter), used as power switching elements, are presented. The unit structure shown in FIGS. 1 and 2 is used, and a multichannel structure comprising a plurality of these unit structures connected in parallel is also used. Here we describe an n-channel SIT in which electrons, for example, form a main current. The unit structure is formed by providing a semiconductor substrate 1 comprising an n.sup.+ -type semiconductor layer with an n.sup.- -type semiconductor layer formed on it. An elongated ring region is formed on the surface of the n.sup.- -type layer, and this is surrounded by a p.sup.+ -type gate region 2. An n.sup.+ -type source region 3 is formed inside the elongated ring region. The minus polarity of a source of the main current is connected to the source region 3, and the n.sup.+ layer forming the other surface area of the semiconductor substrate 1 provides a drain region 4 to which is connected the plus polarity of a source of the main current. The potential of the channel region immediately beneath the source region 3 is lowered by positive holes injected into it from the gate region 4, and electron injection from the source region 3 is induced, thus producing a transistor operation (switching operation).
However, there is the problem that in the surface-gate-type SIT described above, the area of the source region is unnecessarily large, and when a gate current flows, the density of positive holes stored immediately below the source region 3 is lowered, thereby causing a low current amplification factor h.sub.FS (I.sub.D /I.sub.G) of drain current I.sub.D to gate current I.sub.G.